The present invention relates to an improved heat storage system, and more particularly, to a such a system which utilizing phase change materials to repeatedly absorb, store, and release thermal energy.
A great deal of interest exists in phase change thermal energy storage systems because of the inherent ability of phase change materials to store, absorb and release large quantities of heat to their surrounding environment as temperatures drop below or exceed predetermined levels. These systems are particularly of interest in architectural and building trades where climate control and its concomitant energy consumption is one of the principal considerations in building design and material selection.
A variety of building materials and techniques which incorporate phase change materials have previously been used to conserve heat or cool and thereby reduce energy costs. For example, phase change materials have been incorporated into concrete such that energy in excess of that necessary to obtain comfort conditions is inherently absorbed and released as required to maintain the comfort range. Thus, in winter months, phase change materials incorporated into the concrete floors or walls of buildings absorb and store solar energy during daylight hours and release it to the interior at night as temperatures decrease. In the summer months, the same phase change materials, due to their thermostatic character, conserve coolness by absorbing cool from the night air and releasing it during the day.
In recent years, it has been recognized that phase change materials may also be utilized in heat pumps and/or air conditioning systems. For example, Rafalovich et al., U.S. Pat. No. 5,507,337 teach a heat pump and air conditioning system utilizing a refrigerant circuit, a heat exchanger, and a thermal storage device including a first and second phase change materials having different melt temperatures. Camp, U.S. Pat. No. 4,169,554 teaches a solar powered heating and cooling system incorporating a heat pump which utilizes phase change materials such as waxes or paraffins to absorb and store heat energy. Salyer, U.S. Pat. No. 5,765,389 teaches a cooling unit including a heat exchange unit which contains a phase change material.
As water in the liquid-to-vapor phase change is one of the most energetic thermal energy storage materials known (545 cal/g), there is a strong desire to use the heat of vaporization and crystallization of water over other phase change materials since its heat of vaporization is significantly higher. It would be desirable to be able to condense the steam generated in the liquid-to-vapor transition and use the high heat of condensation for heating applications. However, the large change in volume which accompanies the change in state of water from a liquid to a gas necessitates the use of large, pressure-resistant containers which are heavy and expensive.
Accordingly, there is still a need in the art for a low cost, efficient system for storing and transport of large amounts of thermal energy which avoids pressure build-up.